Girlfriend happy.....under false pretenses.....?

Yay, my girl and I are getting on like two peas in a pod once more. After a very emotional discussion, I tell her of a gift I had hidden for her. She finds it and is absolutely ecstatic. Problem is, she thinks its more than it is. A cd, she thought I had gotten autographed. It's not autographed but a print. The magical part is that it was dated a month ago and she thought I was so damn smart cos she only told me a week ago. I wasn't sure why she freaked out so bad till the day after our wonderful night together. I got a surprise and she had the glow and movements of a girl in love once again. I don't want to wreck this joy and good feelings especially after being so close to breakup. How do I approach this? I'm telling her cos I don't want this biting me in the butt later. I don't want that love in her eyes to fade away in disappointment tho.... I don't want her to feel stupid either. I wish I figured it out sooner or I'd have told her up up front. :-(